By means of optical scanners it is possible to obtain an electronic representation of printed information on a document. The electronic representation is obtained by detecting light intensities reflected from small picture elements (pixels) arranged in a line covering the spatial extent of the document subject to scanning. The light intensities from the pixels on the document are detected by means of some type of pixel intensity detectors providing electrical signals that comprises intensity levels. In order to scan the document efficiently and sufficiently fast the light intensities are detected by some type of line detector (e.g. using charge coupled devices, CCD's) having an array of pixel intensity detectors detecting the intensity of multiple pixels on the document in a single shot. Typically, the pixel intensity detectors are of some type of semiconductor material and are arranged on a common substrate. Despite the fact that the intensity detectors are arranged on a common substrate and that a document subject to scanning is completely uniformly coloured, the intensity detectors do not provide exactly the same intensity levels. This may be due to different sensitivities of the individual intensity detectors and due to the camera detecting light from the original through an optical system having different light paths for each pixel. The sensitivities of the individual intensity detectors change very slowly over time under normal operation conditions. To conclude, each intensity detector in the camera converts the intensities on the original to intensity levels by means of pixel-to-pixel varying coefficients which can vary considerably from pixel to pixel but is substantially constant in time for each individual pixel.
Typically, the document is illuminated by some type of light source e.g. a fluorescent light tube when the document is scanned. However, the light source will not illuminate the field of view of the line detector on the document uniformly. By field of view is understood pixels of which the line detector is capable to detect intensity levels when the camera and the document are held fixed relative Lo each other. The non-uniform illumination will distort the detection of the information on the document and thereby also distort the electronical representation of the document. This effect will affect the response of the individual pixel intensity detectors, however this contribution to the response will vary rather smoothly from one pixel to its neighbouring pixel ie across pixels. This contribution to the response should not be confused with the sensitivities for the individual pixels. The illumination of the document will change significantly as a function of temperature and time.
In the prior art the detected intensity levels are corrected by a set of amplification values in order to compensate for the non-uniform illumination. In optical scanners e.g. in the so-called roller-fed scanners where the document is moved past the line detector in order to scan the entire document with a single line detector, the amplification values are obtained by scanning the line profile data of a uniform white background just before a document subject to scanning is moved in front of the line detector. In such scanners the document is also moved past and, in particular, in contact with the background in order to support the document under the line detector during scanning. Therefore, the background will deteriorate rather quickly, which means that e.g. scratches may appear in the surface. Further, particles coming off the document due to the mechanical moving of the document results in that the background will become dirty. In order to remove spikes in the line profile data arising from scratches and dust in/on the background i.e. to provide reliable amplification values, line profile data in the form of intensity levels arising from scanning the background is soothed or low-pass filtered.
So, there exists a problem in the prior art that the information regarding the individual and across pixels varying sensitivities is diminished due to the low-pass filtering. When monochromatic scanners are considered this results in the minimal obtainable signal-to-noise ratio in the electronic representation of the document being determined by the variation in the pixel-specific sensitivities. Additionally, when multicolour scanners are considered, colour distortions will appear. The colour distortions will be seen as distinct single colour strikes when the electronical representation of the document is viewed. This is because the colour representation is obtained by detecting the intensity at a number of different colours e.g. the colours red, green, and blue by means of an individual detector for each colour. An example can be considered: if a document having a uniform white surface is scanned and the intensity level provided by the line detector for a specific pixel on a detector detecting red intensity information is, wrongly, relatively large compared to the detected intensity levels of the colours green and blue, then this pixel will appear as having a red tint although it should have been white.